1. Field of the Invention
The present invention relates to the field of data dependent acquisition in a mass spectrometer, and more particularly to the field of tandem mass spectrometry.
2. Discussion of the Related Art
Mass spectrometers are often coupled with chromatography systems in order to identify and characterize eluting species from a test sample. In such a coupled system, the eluent is ionized and a series of mass spectral scans are obtained at specified time intervals for subsequent data analysis. As the test sample may contain many species or compounds, it is often desirable to be able to automatically determine or identify species or compounds of interest as they elute and perform tandem mass spectrometry analysis to characterize them.
Tandem mass spectrometry is a mode of operation that utilizes multiple stages of mass analysis with a collision or reaction process between each stage of mass analysis. The coupling of multiple stages of mass analysis provides the ability to determine or identify species or compounds of interest by providing additional information on the fragmentation or reaction characteristics of the compound. Tandem mass spectrometry having two stages of mass analysis is typically referred to as mass spectrometry/mass spectrometry (MS/MS). In data dependent mode, the eluting sample is automatically selected for further analysis by MS/MS when the signal intensity of a mass spectral peak is above a user specified intensity. But, direct intensity based triggering is far from ideal, for several reasons. First, high baselines will cause triggering at all masses. Second, tandem mass spectra will be collected as soon as an eluting chromatographic peak exceeds the threshold value, and not at the ideal point, which is at the top of the chromatographic peak where it is at its greatest intensity. Collecting a sample for MS/MS at the beginning of the elution of the chromatographic peak produces a lower quality spectrum, due to a limited number of sample ions being combined with a high percentage of background contaminants. For ion accumulating mass spectrometers, which use automatic gain control, such as quadrupole ion traps and Fourier transform mass spectrometers, performing MS/MS at the beginning of elution also requires the largest amount of time, thus slowing analysis. Third, direct intensity based triggering results in redundant MS/MS acquisition of compounds during the entire elution time.
United States Published Patent Application US 2004/0251409 describes a method of dynamic background signal exclusion in chromatography/mass spectrometry data-dependent data acquisition to detect species eluting at a low level of concentration that elute simultaneously with a number of other major components by identifying ions having a fast rising mass signal. The ions having the fastest rising mass signal may be identified by subtracting a previously acquired mass spectrum, or an average of previously acquired mass spectra, from the current mass spectrum. The disadvantage of taking a sample of the eluted ionized species at the fastest rising mass signal is that MS/MS can often be triggered on noise spikes in the spectrum, caused by either charged droplets from the ion source or electrical noise in the detection system.